/*
 * Windows networking abstraction.
 *
 * For the IPv6 code in here I am indebted to Jeroen Massar and
 * unfix.org.
 */

#include <winsock2.h> /* need to put this first, for winelib builds */

#include <stdio.h>
#include <stdlib.h>
#include <assert.h>

#define DEFINE_PLUG_METHOD_MACROS
#define NEED_DECLARATION_OF_SELECT /* in order to initialise it */

#include "putty.h"
#include "network.h"
#include "tree234.h"

#include <ws2tcpip.h>

#ifndef NO_IPV6
#ifdef __clang__
#pragma clang diagnostic push
#pragma clang diagnostic ignored "-Wmissing-braces"
#endif
const struct in6_addr in6addr_any = IN6ADDR_ANY_INIT;
const struct in6_addr in6addr_loopback = IN6ADDR_LOOPBACK_INIT;
#ifdef __clang__
#pragma clang diagnostic pop
#endif
#endif

#define ipv4_is_loopback(addr)                                                 \
  ((p_ntohl(addr.s_addr) & 0xFF000000L) == 0x7F000000L)

/*
 * We used to typedef struct Socket_tag *Socket.
 *
 * Since we have made the networking abstraction slightly more
 * abstract, Socket no longer means a tcp socket (it could mean
 * an ssl socket).  So now we must use Actual_Socket when we know
 * we are talking about a tcp socket.
 */
typedef struct Socket_tag *Actual_Socket;

/*
 * Mutable state that goes with a SockAddr: stores information
 * about where in the list of candidate IP(v*) addresses we've
 * currently got to.
 */
typedef struct SockAddrStep_tag SockAddrStep;
struct SockAddrStep_tag {
#ifndef NO_IPV6
  struct addrinfo *ai; /* steps along addr->ais */
#endif
  int curraddr;
};

struct Socket_tag {
  const struct socket_function_table *fn;
  /* the above variable absolutely *must* be the first in this structure */
  const char *error;
  SOCKET s;
  Plug plug;
  bufchain output_data;
  int connected;
  int writable;
  int frozen;          /* this causes readability notifications to be ignored */
  int frozen_readable; /* this means we missed at least one readability
                        * notification while we were frozen */
  int localhost_only;  /* for listening sockets */
  char oobdata[1];
  int sending_oob;
  int oobinline, nodelay, keepalive, privport;
  enum
  {
    EOF_NO,
    EOF_PENDING,
    EOF_SENT
  } outgoingeof;
  SockAddr addr;
  SockAddrStep step;
  int port;
  int pending_error; /* in case send() returns error */
  /*
   * We sometimes need pairs of Socket structures to be linked:
   * if we are listening on the same IPv6 and v4 port, for
   * example. So here we define `parent' and `child' pointers to
   * track this link.
   */
  Actual_Socket parent, child;
};

struct SockAddr_tag {
  int refcount;
  char *error;
  int resolved;
  int namedpipe; /* indicates that this SockAddr is phony, holding a Windows
                  * named pipe pathname instead of a network address */
#ifndef NO_IPV6
  struct addrinfo *ais; /* Addresses IPv6 style. */
#endif
  unsigned long *addresses; /* Addresses IPv4 style. */
  int naddresses;
  char hostname[512]; /* Store an unresolved host name. */
};

/*
 * Which address family this address belongs to. AF_INET for IPv4;
 * AF_INET6 for IPv6; AF_UNSPEC indicates that name resolution has
 * not been done and a simple host name is held in this SockAddr
 * structure.
 */
#ifndef NO_IPV6
#define SOCKADDR_FAMILY(addr, step)                                            \
  (!(addr)->resolved ? AF_UNSPEC : (step).ai ? (step).ai->ai_family : AF_INET)
#else
#define SOCKADDR_FAMILY(addr, step) (!(addr)->resolved ? AF_UNSPEC : AF_INET)
#endif

/*
 * Start a SockAddrStep structure to step through multiple
 * addresses.
 */
#ifndef NO_IPV6
#define START_STEP(addr, step) ((step).ai = (addr)->ais, (step).curraddr = 0)
#else
#define START_STEP(addr, step) ((step).curraddr = 0)
#endif

static tree234 *sktree;

static int cmpfortree(void *av, void *bv)
{
  Actual_Socket a = (Actual_Socket)av, b = (Actual_Socket)bv;
  unsigned long as = (unsigned long)a->s, bs = (unsigned long)b->s;
  if (as < bs)
    return -1;
  if (as > bs)
    return +1;
  if (a < b)
    return -1;
  if (a > b)
    return +1;
  return 0;
}

static int cmpforsearch(void *av, void *bv)
{
  Actual_Socket b = (Actual_Socket)bv;
  uintptr_t as = (uintptr_t)av, bs = (uintptr_t)b->s;
  if (as < bs)
    return -1;
  if (as > bs)
    return +1;
  return 0;
}

DECL_WINDOWS_FUNCTION(static, int, WSAStartup, (WORD, LPWSADATA));
DECL_WINDOWS_FUNCTION(static, int, WSACleanup, (void));
DECL_WINDOWS_FUNCTION(static, int, closesocket, (SOCKET));
DECL_WINDOWS_FUNCTION(static, u_long, ntohl, (u_long));
DECL_WINDOWS_FUNCTION(static, u_long, htonl, (u_long));
DECL_WINDOWS_FUNCTION(static, u_short, htons, (u_short));
DECL_WINDOWS_FUNCTION(static, u_short, ntohs, (u_short));
DECL_WINDOWS_FUNCTION(static, int, gethostname, (char *, int));
DECL_WINDOWS_FUNCTION(static,
                      struct hostent FAR *,
                      gethostbyname,
                      (const char FAR *));
DECL_WINDOWS_FUNCTION(static,
                      struct servent FAR *,
                      getservbyname,
                      (const char FAR *, const char FAR *));
DECL_WINDOWS_FUNCTION(static, unsigned long, inet_addr, (const char FAR *));
DECL_WINDOWS_FUNCTION(static, char FAR *, inet_ntoa, (struct in_addr));
DECL_WINDOWS_FUNCTION(static,
                      const char FAR *,
                      inet_ntop,
                      (int, void FAR *, char *, size_t));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      connect,
                      (SOCKET, const struct sockaddr FAR *, int));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      bind,
                      (SOCKET, const struct sockaddr FAR *, int));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      setsockopt,
                      (SOCKET, int, int, const char FAR *, int));
DECL_WINDOWS_FUNCTION(static, SOCKET, socket, (int, int, int));
DECL_WINDOWS_FUNCTION(static, int, listen, (SOCKET, int));
DECL_WINDOWS_FUNCTION(static, int, send, (SOCKET, const char FAR *, int, int));
DECL_WINDOWS_FUNCTION(static, int, shutdown, (SOCKET, int));
DECL_WINDOWS_FUNCTION(static, int, ioctlsocket, (SOCKET, long, u_long FAR *));
DECL_WINDOWS_FUNCTION(static,
                      SOCKET,
                      accept,
                      (SOCKET, struct sockaddr FAR *, int FAR *));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      getpeername,
                      (SOCKET, struct sockaddr FAR *, int FAR *));
DECL_WINDOWS_FUNCTION(static, int, recv, (SOCKET, char FAR *, int, int));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      WSAIoctl,
                      (SOCKET,
                       DWORD,
                       LPVOID,
                       DWORD,
                       LPVOID,
                       DWORD,
                       LPDWORD,
                       LPWSAOVERLAPPED,
                       LPWSAOVERLAPPED_COMPLETION_ROUTINE));
#ifndef NO_IPV6
DECL_WINDOWS_FUNCTION(static,
                      int,
                      getaddrinfo,
                      (const char *nodename,
                       const char *servname,
                       const struct addrinfo *hints,
                       struct addrinfo **res));
DECL_WINDOWS_FUNCTION(static, void, freeaddrinfo, (struct addrinfo * res));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      getnameinfo,
                      (const struct sockaddr FAR *sa,
                       socklen_t salen,
                       char FAR *host,
                       size_t hostlen,
                       char FAR *serv,
                       size_t servlen,
                       int flags));
DECL_WINDOWS_FUNCTION(static, char *, gai_strerror, (int ecode));
DECL_WINDOWS_FUNCTION(static,
                      int,
                      WSAAddressToStringA,
                      (LPSOCKADDR, DWORD, LPWSAPROTOCOL_INFO, LPSTR, LPDWORD));
#endif

static HMODULE winsock_module = NULL;
static WSADATA wsadata;
#ifndef NO_IPV6
static HMODULE winsock2_module = NULL;
static HMODULE wship6_module = NULL;
#endif

int sk_startup(int hi, int lo)
{
  WORD winsock_ver;

  winsock_ver = MAKEWORD(hi, lo);

  if (p_WSAStartup(winsock_ver, &wsadata)) {
    return FALSE;
  }

  if (LOBYTE(wsadata.wVersion) != LOBYTE(winsock_ver)) {
    return FALSE;
  }

#ifdef NET_SETUP_DIAGNOSTICS
  {
    char buf[80];
    sprintf(buf, "Using WinSock %d.%d", hi, lo);
    logevent(NULL, buf);
  }
#endif
  return TRUE;
}

/* Actually define this function pointer, which won't have been
 * defined alongside all the others by PUTTY_DO_GLOBALS because of the
 * annoying winelib header-ordering issue. (See comment in winstuff.h.) */
DECL_WINDOWS_FUNCTION(/* empty */,
                      int,
                      select,
                      (int,
                       fd_set FAR *,
                       fd_set FAR *,
                       fd_set FAR *,
                       const struct timeval FAR *));

void sk_init(void)
{
#ifndef NO_IPV6
  winsock2_module =
#endif
      winsock_module = load_system32_dll("ws2_32.dll");
  if (!winsock_module) {
    winsock_module = load_system32_dll("wsock32.dll");
  }
  if (!winsock_module)
    fatalbox("Unable to load any WinSock library");

#ifndef NO_IPV6
  /* Check if we have getaddrinfo in Winsock */
  if (GetProcAddress(winsock_module, "getaddrinfo") != NULL) {
#ifdef NET_SETUP_DIAGNOSTICS
    logevent(NULL, "Native WinSock IPv6 support detected");
#endif
    GET_WINDOWS_FUNCTION(winsock_module, getaddrinfo);
    GET_WINDOWS_FUNCTION(winsock_module, freeaddrinfo);
    GET_WINDOWS_FUNCTION(winsock_module, getnameinfo);
    /* This function would fail its type-check if we did one,
     * because the VS header file provides an inline definition
     * which is __cdecl instead of WINAPI. */
    GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, gai_strerror);
  } else {
    /* Fall back to wship6.dll for Windows 2000 */
    wship6_module = load_system32_dll("wship6.dll");
    if (wship6_module) {
#ifdef NET_SETUP_DIAGNOSTICS
      logevent(NULL, "WSH IPv6 support detected");
#endif
      GET_WINDOWS_FUNCTION(wship6_module, getaddrinfo);
      GET_WINDOWS_FUNCTION(wship6_module, freeaddrinfo);
      GET_WINDOWS_FUNCTION(wship6_module, getnameinfo);
      /* See comment above about type check */
      GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, gai_strerror);
    } else {
#ifdef NET_SETUP_DIAGNOSTICS
      logevent(NULL, "No IPv6 support detected");
#endif
    }
  }
  GET_WINDOWS_FUNCTION(winsock2_module, WSAAddressToStringA);
#else
#ifdef NET_SETUP_DIAGNOSTICS
  logevent(NULL, "PuTTY was built without IPv6 support");
#endif
#endif

  GET_WINDOWS_FUNCTION(winsock_module, WSAAsyncSelect);
  GET_WINDOWS_FUNCTION(winsock_module, WSAEventSelect);
  GET_WINDOWS_FUNCTION(winsock_module, select);
  GET_WINDOWS_FUNCTION(winsock_module, WSAGetLastError);
  GET_WINDOWS_FUNCTION(winsock_module, WSAEnumNetworkEvents);
  GET_WINDOWS_FUNCTION(winsock_module, WSAStartup);
  GET_WINDOWS_FUNCTION(winsock_module, WSACleanup);
  GET_WINDOWS_FUNCTION(winsock_module, closesocket);
#ifndef COVERITY
  GET_WINDOWS_FUNCTION(winsock_module, ntohl);
  GET_WINDOWS_FUNCTION(winsock_module, htonl);
  GET_WINDOWS_FUNCTION(winsock_module, htons);
  GET_WINDOWS_FUNCTION(winsock_module, ntohs);
  GET_WINDOWS_FUNCTION(winsock_module, gethostname);
#else
  /* The toolchain I use for Windows Coverity builds doesn't know
   * the type signatures of these */
  GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, ntohl);
  GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, htonl);
  GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, htons);
  GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, ntohs);
  GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, gethostname);
#endif
  GET_WINDOWS_FUNCTION(winsock_module, gethostbyname);
  GET_WINDOWS_FUNCTION(winsock_module, getservbyname);
  GET_WINDOWS_FUNCTION(winsock_module, inet_addr);
  GET_WINDOWS_FUNCTION(winsock_module, inet_ntoa);
#if (defined _MSC_VER && _MSC_VER < 1900) || defined __MINGW32__
  /* Older Visual Studio, and MinGW as of Ubuntu 16.04, don't know
   * about this function at all, so can't type-check it */
  GET_WINDOWS_FUNCTION_NO_TYPECHECK(winsock_module, inet_ntop);
#else
  GET_WINDOWS_FUNCTION(winsock_module, inet_ntop);
#endif
  GET_WINDOWS_FUNCTION(winsock_module, connect);
  GET_WINDOWS_FUNCTION(winsock_module, bind);
  GET_WINDOWS_FUNCTION(winsock_module, setsockopt);
  GET_WINDOWS_FUNCTION(winsock_module, socket);
  GET_WINDOWS_FUNCTION(winsock_module, listen);
  GET_WINDOWS_FUNCTION(winsock_module, send);
  GET_WINDOWS_FUNCTION(winsock_module, shutdown);
  GET_WINDOWS_FUNCTION(winsock_module, ioctlsocket);
  GET_WINDOWS_FUNCTION(winsock_module, accept);
  GET_WINDOWS_FUNCTION(winsock_module, getpeername);
  GET_WINDOWS_FUNCTION(winsock_module, recv);
  GET_WINDOWS_FUNCTION(winsock_module, WSAIoctl);

  /* Try to get the best WinSock version we can get */
  if (!sk_startup(2, 2) && !sk_startup(2, 0) && !sk_startup(1, 1)) {
    fatalbox("Unable to initialise WinSock");
  }

  sktree = newtree234(cmpfortree);
}

void sk_cleanup(void)
{
  Actual_Socket s;
  int i;

  if (sktree) {
    for (i = 0; (s = index234(sktree, i)) != NULL; i++) {
      p_closesocket(s->s);
    }
    freetree234(sktree);
    sktree = NULL;
  }

  if (p_WSACleanup)
    p_WSACleanup();
  if (winsock_module)
    FreeLibrary(winsock_module);
#ifndef NO_IPV6
  if (wship6_module)
    FreeLibrary(wship6_module);
#endif
}

struct errstring {
  int error;
  char *text;
};

static int errstring_find(void *av, void *bv)
{
  int *a = (int *)av;
  struct errstring *b = (struct errstring *)bv;
  if (*a < b->error)
    return -1;
  if (*a > b->error)
    return +1;
  return 0;
}
static int errstring_compare(void *av, void *bv)
{
  struct errstring *a = (struct errstring *)av;
  return errstring_find(&a->error, bv);
}

static tree234 *errstrings = NULL;

const char *winsock_error_string(int error)
{
  const char prefix[] = "Network error: ";
  struct errstring *es;

  /*
   * Error codes we know about and have historically had reasonably
   * sensible error messages for.
   */
  switch (error) {
  case WSAEACCES:
    return "Network error: Permission denied";
  case WSAEADDRINUSE:
    return "Network error: Address already in use";
  case WSAEADDRNOTAVAIL:
    return "Network error: Cannot assign requested address";
  case WSAEAFNOSUPPORT:
    return "Network error: Address family not supported by protocol family";
  case WSAEALREADY:
    return "Network error: Operation already in progress";
  case WSAECONNABORTED:
    return "Network error: Software caused connection abort";
  case WSAECONNREFUSED:
    return "Network error: Connection refused";
  case WSAECONNRESET:
    return "Network error: Connection reset by peer";
  case WSAEDESTADDRREQ:
    return "Network error: Destination address required";
  case WSAEFAULT:
    return "Network error: Bad address";
  case WSAEHOSTDOWN:
    return "Network error: Host is down";
  case WSAEHOSTUNREACH:
    return "Network error: No route to host";
  case WSAEINPROGRESS:
    return "Network error: Operation now in progress";
  case WSAEINTR:
    return "Network error: Interrupted function call";
  case WSAEINVAL:
    return "Network error: Invalid argument";
  case WSAEISCONN:
    return "Network error: Socket is already connected";
  case WSAEMFILE:
    return "Network error: Too many open files";
  case WSAEMSGSIZE:
    return "Network error: Message too long";
  case WSAENETDOWN:
    return "Network error: Network is down";
  case WSAENETRESET:
    return "Network error: Network dropped connection on reset";
  case WSAENETUNREACH:
    return "Network error: Network is unreachable";
  case WSAENOBUFS:
    return "Network error: No buffer space available";
  case WSAENOPROTOOPT:
    return "Network error: Bad protocol option";
  case WSAENOTCONN:
    return "Network error: Socket is not connected";
  case WSAENOTSOCK:
    return "Network error: Socket operation on non-socket";
  case WSAEOPNOTSUPP:
    return "Network error: Operation not supported";
  case WSAEPFNOSUPPORT:
    return "Network error: Protocol family not supported";
  case WSAEPROCLIM:
    return "Network error: Too many processes";
  case WSAEPROTONOSUPPORT:
    return "Network error: Protocol not supported";
  case WSAEPROTOTYPE:
    return "Network error: Protocol wrong type for socket";
  case WSAESHUTDOWN:
    return "Network error: Cannot send after socket shutdown";
  case WSAESOCKTNOSUPPORT:
    return "Network error: Socket type not supported";
  case WSAETIMEDOUT:
    return "Network error: Connection timed out";
  case WSAEWOULDBLOCK:
    return "Network error: Resource temporarily unavailable";
  case WSAEDISCON:
    return "Network error: Graceful shutdown in progress";
  }

  /*
   * Generic code to handle any other error.
   *
   * Slightly nasty hack here: we want to return a static string
   * which the caller will never have to worry about freeing, but on
   * the other hand if we call FormatMessage to get it then it will
   * want to either allocate a buffer or write into one we own.
   *
   * So what we do is to maintain a tree234 of error strings we've
   * already used. New ones are allocated from the heap, but then
   * put in this tree and kept forever.
   */

  if (!errstrings)
    errstrings = newtree234(errstring_compare);

  es = find234(errstrings, &error, errstring_find);

  if (!es) {
    int bufsize, bufused;

    es = snew(struct errstring);
    es->error = error;
    /* maximum size for FormatMessage is 64K */
    bufsize = 65535 + sizeof(prefix);
    es->text = snewn(bufsize, char);
    strcpy(es->text, prefix);
    bufused = strlen(es->text);
    if (!FormatMessage(
            (FORMAT_MESSAGE_FROM_SYSTEM | FORMAT_MESSAGE_IGNORE_INSERTS),
            NULL,
            error,
            MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),
            es->text + bufused,
            bufsize - bufused,
            NULL)) {
      sprintf(es->text + bufused,
              "Windows error code %d (and FormatMessage returned %u)",
              error,
              (unsigned int)GetLastError());
    } else {
      int len = strlen(es->text);
      if (len > 0 && es->text[len - 1] == '\n')
        es->text[len - 1] = '\0';
    }
    es->text = sresize(es->text, strlen(es->text) + 1, char);
    add234(errstrings, es);
  }

  return es->text;
}

SockAddr sk_namelookup(const char *host,
                       char **canonicalname,
                       int address_family)
{
  SockAddr ret = snew(struct SockAddr_tag);
  unsigned long a;
  char realhost[8192];
  int hint_family;

  /* Default to IPv4. */
  hint_family = (address_family == ADDRTYPE_IPV4
                     ? AF_INET
                     :
#ifndef NO_IPV6
                     address_family == ADDRTYPE_IPV6 ? AF_INET6 :
#endif
                                                     AF_UNSPEC);

  /* Clear the structure and default to IPv4. */
  memset(ret, 0, sizeof(struct SockAddr_tag));
#ifndef NO_IPV6
  ret->ais = NULL;
#endif
  ret->namedpipe = FALSE;
  ret->addresses = NULL;
  ret->resolved = FALSE;
  ret->refcount = 1;
  *realhost = '\0';

  if ((a = p_inet_addr(host)) == (unsigned long)INADDR_NONE) {
    struct hostent *h = NULL;
    int err = 0;
#ifndef NO_IPV6
    /*
     * Use getaddrinfo when it's available
     */
    if (p_getaddrinfo) {
      struct addrinfo hints;
#ifdef NET_SETUP_DIAGNOSTICS
      logevent(NULL, "Using getaddrinfo() for resolving");
#endif
      memset(&hints, 0, sizeof(hints));
      hints.ai_family = hint_family;
      hints.ai_flags = AI_CANONNAME;
      {
        /* strip [] on IPv6 address literals */
        char *trimmed_host = host_strduptrim(host);
        err = p_getaddrinfo(trimmed_host, NULL, &hints, &ret->ais);
        sfree(trimmed_host);
      }
      if (err == 0)
        ret->resolved = TRUE;
    } else
#endif
    {
#ifdef NET_SETUP_DIAGNOSTICS
      logevent(NULL, "Using gethostbyname() for resolving");
#endif
      /*
       * Otherwise use the IPv4-only gethostbyname...
       * (NOTE: we don't use gethostbyname as a fallback!)
       */
      if ((h = p_gethostbyname(host)))
        ret->resolved = TRUE;
      else
        err = p_WSAGetLastError();
    }

    if (!ret->resolved) {
      ret->error =
          (err == WSAENETDOWN
               ? "Network is down"
               : err == WSAHOST_NOT_FOUND
                     ? "Host does not exist"
                     : err == WSATRY_AGAIN ? "Host not found" :
#ifndef NO_IPV6
                                           p_getaddrinfo && p_gai_strerror
                                               ? p_gai_strerror(err)
                                               :
#endif
                                               "gethostbyname: unknown error");
    } else {
      ret->error = NULL;

#ifndef NO_IPV6
      /* If we got an address info use that... */
      if (ret->ais) {
        /* Are we in IPv4 fallback mode? */
        /* We put the IPv4 address into the a variable so we can further-on use
         * the IPv4 code... */
        if (ret->ais->ai_family == AF_INET)
          memcpy(&a,
                 (char *)&((SOCKADDR_IN *)ret->ais->ai_addr)->sin_addr,
                 sizeof(a));

        if (ret->ais->ai_canonname)
          strncpy(realhost, ret->ais->ai_canonname, lenof(realhost));
        else
          strncpy(realhost, host, lenof(realhost));
      }
      /* We used the IPv4-only gethostbyname()... */
      else
#endif
      {
        int n;
        for (n = 0; h->h_addr_list[n]; n++)
          ;
        ret->addresses = snewn(n, unsigned long);
        ret->naddresses = n;
        for (n = 0; n < ret->naddresses; n++) {
          memcpy(&a, h->h_addr_list[n], sizeof(a));
          ret->addresses[n] = p_ntohl(a);
        }
        memcpy(&a, h->h_addr, sizeof(a));
        /* This way we are always sure the h->h_name is valid :) */
        strncpy(realhost, h->h_name, sizeof(realhost));
      }
    }
  } else {
    /*
     * This must be a numeric IPv4 address because it caused a
     * success return from inet_addr.
     */
    ret->addresses = snewn(1, unsigned long);
    ret->naddresses = 1;
    ret->addresses[0] = p_ntohl(a);
    ret->resolved = TRUE;
    strncpy(realhost, host, sizeof(realhost));
  }
  realhost[lenof(realhost) - 1] = '\0';
  *canonicalname = snewn(1 + strlen(realhost), char);
  strcpy(*canonicalname, realhost);
  return ret;
}

SockAddr sk_nonamelookup(const char *host)
{
  SockAddr ret = snew(struct SockAddr_tag);
  ret->error = NULL;
  ret->resolved = FALSE;
#ifndef NO_IPV6
  ret->ais = NULL;
#endif
  ret->namedpipe = FALSE;
  ret->addresses = NULL;
  ret->naddresses = 0;
  ret->refcount = 1;
  strncpy(ret->hostname, host, lenof(ret->hostname));
  ret->hostname[lenof(ret->hostname) - 1] = '\0';
  return ret;
}

SockAddr sk_namedpipe_addr(const char *pipename)
{
  SockAddr ret = snew(struct SockAddr_tag);
  ret->error = NULL;
  ret->resolved = FALSE;
#ifndef NO_IPV6
  ret->ais = NULL;
#endif
  ret->namedpipe = TRUE;
  ret->addresses = NULL;
  ret->naddresses = 0;
  ret->refcount = 1;
  strncpy(ret->hostname, pipename, lenof(ret->hostname));
  ret->hostname[lenof(ret->hostname) - 1] = '\0';
  return ret;
}

int sk_nextaddr(SockAddr addr, SockAddrStep *step)
{
#ifndef NO_IPV6
  if (step->ai) {
    if (step->ai->ai_next) {
      step->ai = step->ai->ai_next;
      return TRUE;
    } else
      return FALSE;
  }
#endif
  if (step->curraddr + 1 < addr->naddresses) {
    step->curraddr++;
    return TRUE;
  } else {
    return FALSE;
  }
}

void sk_getaddr(SockAddr addr, char *buf, int buflen)
{
  SockAddrStep step;
  START_STEP(addr, step);

#ifndef NO_IPV6
  if (step.ai) {
    int err = 0;
    if (p_WSAAddressToStringA) {
      DWORD dwbuflen = buflen;
      err = p_WSAAddressToStringA(
          step.ai->ai_addr, step.ai->ai_addrlen, NULL, buf, &dwbuflen);
    } else
      err = -1;
    if (err) {
      strncpy(buf, addr->hostname, buflen);
      if (!buf[0])
        strncpy(buf, "<unknown>", buflen);
      buf[buflen - 1] = '\0';
    }
  } else
#endif
      if (SOCKADDR_FAMILY(addr, step) == AF_INET) {
    struct in_addr a;
    assert(addr->addresses && step.curraddr < addr->naddresses);
    a.s_addr = p_htonl(addr->addresses[step.curraddr]);
    strncpy(buf, p_inet_ntoa(a), buflen);
    buf[buflen - 1] = '\0';
  } else {
    strncpy(buf, addr->hostname, buflen);
    buf[buflen - 1] = '\0';
  }
}

/*
 * This constructs a SockAddr that points at one specific sub-address
 * of a parent SockAddr. The returned SockAddr does not own all its
 * own memory: it points into the old one's data structures, so it
 * MUST NOT be used after the old one is freed, and it MUST NOT be
 * passed to sk_addr_free. (The latter is why it's returned by value
 * rather than dynamically allocated - that should clue in anyone
 * writing a call to it that something is weird about it.)
 */
static struct SockAddr_tag sk_extractaddr_tmp(SockAddr addr,
                                              const SockAddrStep *step)
{
  struct SockAddr_tag toret;
  toret = *addr; /* structure copy */
  toret.refcount = 1;

#ifndef NO_IPV6
  toret.ais = step->ai;
#endif
  if (SOCKADDR_FAMILY(addr, *step) == AF_INET
#ifndef NO_IPV6
      && !toret.ais
#endif
  )
    toret.addresses += step->curraddr;

  return toret;
}

int sk_addr_needs_port(SockAddr addr)
{
  return addr->namedpipe ? FALSE : TRUE;
}

int sk_hostname_is_local(const char *name)
{
  return !strcmp(name, "localhost") || !strcmp(name, "::1") ||
         !strncmp(name, "127.", 4);
}

static INTERFACE_INFO local_interfaces[16];
static int n_local_interfaces; /* 0=not yet, -1=failed, >0=number */

static int ipv4_is_local_addr(struct in_addr addr)
{
  if (ipv4_is_loopback(addr))
    return 1; /* loopback addresses are local */
  if (!n_local_interfaces) {
    SOCKET s = p_socket(AF_INET, SOCK_DGRAM, 0);
    DWORD retbytes;

    SetHandleInformation((HANDLE)s, HANDLE_FLAG_INHERIT, 0);

    if (p_WSAIoctl && p_WSAIoctl(s,
                                 SIO_GET_INTERFACE_LIST,
                                 NULL,
                                 0,
                                 local_interfaces,
                                 sizeof(local_interfaces),
                                 &retbytes,
                                 NULL,
                                 NULL) == 0)
      n_local_interfaces = retbytes / sizeof(INTERFACE_INFO);
    else
      logevent(NULL, "Unable to get list of local IP addresses");
  }
  if (n_local_interfaces > 0) {
    int i;
    for (i = 0; i < n_local_interfaces; i++) {
      SOCKADDR_IN *address = (SOCKADDR_IN *)&local_interfaces[i].iiAddress;
      if (address->sin_addr.s_addr == addr.s_addr)
        return 1; /* this address is local */
    }
  }
  return 0; /* this address is not local */
}

int sk_address_is_local(SockAddr addr)
{
  SockAddrStep step;
  int family;
  START_STEP(addr, step);
  family = SOCKADDR_FAMILY(addr, step);

#ifndef NO_IPV6
  if (family == AF_INET6) {
    return IN6_IS_ADDR_LOOPBACK(
        &((const struct sockaddr_in6 *)step.ai->ai_addr)->sin6_addr);
  } else
#endif
      if (family == AF_INET) {
#ifndef NO_IPV6
    if (step.ai) {
      return ipv4_is_local_addr(
          ((struct sockaddr_in *)step.ai->ai_addr)->sin_addr);
    } else
#endif
    {
      struct in_addr a;
      assert(addr->addresses && step.curraddr < addr->naddresses);
      a.s_addr = p_htonl(addr->addresses[step.curraddr]);
      return ipv4_is_local_addr(a);
    }
  } else {
    assert(family == AF_UNSPEC);
    return 0; /* we don't know; assume not */
  }
}

int sk_address_is_special_local(SockAddr addr)
{
  return 0; /* no Unix-domain socket analogue here */
}

int sk_addrtype(SockAddr addr)
{
  SockAddrStep step;
  int family;
  START_STEP(addr, step);
  family = SOCKADDR_FAMILY(addr, step);

  return (family == AF_INET ? ADDRTYPE_IPV4 :
#ifndef NO_IPV6
                            family == AF_INET6 ? ADDRTYPE_IPV6 :
#endif
                                               ADDRTYPE_NAME);
}

void sk_addrcopy(SockAddr addr, char *buf)
{
  SockAddrStep step;
  int family;
  START_STEP(addr, step);
  family = SOCKADDR_FAMILY(addr, step);

  assert(family != AF_UNSPEC);
#ifndef NO_IPV6
  if (step.ai) {
    if (family == AF_INET)
      memcpy(buf,
             &((struct sockaddr_in *)step.ai->ai_addr)->sin_addr,
             sizeof(struct in_addr));
    else if (family == AF_INET6)
      memcpy(buf,
             &((struct sockaddr_in6 *)step.ai->ai_addr)->sin6_addr,
             sizeof(struct in6_addr));
    else
      assert(FALSE);
  } else
#endif
      if (family == AF_INET) {
    struct in_addr a;
    assert(addr->addresses && step.curraddr < addr->naddresses);
    a.s_addr = p_htonl(addr->addresses[step.curraddr]);
    memcpy(buf, (char *)&a.s_addr, 4);
  }
}

void sk_addr_free(SockAddr addr)
{
  if (--addr->refcount > 0)
    return;
#ifndef NO_IPV6
  if (addr->ais && p_freeaddrinfo)
    p_freeaddrinfo(addr->ais);
#endif
  if (addr->addresses)
    sfree(addr->addresses);
  sfree(addr);
}

SockAddr sk_addr_dup(SockAddr addr)
{
  addr->refcount++;
  return addr;
}

static Plug sk_tcp_plug(Socket sock, Plug p)
{
  Actual_Socket s = (Actual_Socket)sock;
  Plug ret = s->plug;
  if (p)
    s->plug = p;
  return ret;
}

static void sk_tcp_flush(Socket s)
{
  /*
   * We send data to the socket as soon as we can anyway,
   * so we don't need to do anything here.  :-)
   */
}

static void sk_tcp_close(Socket s);
static int sk_tcp_write(Socket s, const char *data, int len);
static int sk_tcp_write_oob(Socket s, const char *data, int len);
static void sk_tcp_write_eof(Socket s);
static void sk_tcp_set_frozen(Socket s, int is_frozen);
static const char *sk_tcp_socket_error(Socket s);
static char *sk_tcp_peer_info(Socket s);

extern char *do_select(SOCKET skt, int startup);

static Socket sk_tcp_accept(accept_ctx_t ctx, Plug plug)
{
  static const struct socket_function_table fn_table = {
      sk_tcp_plug,
      sk_tcp_close,
      sk_tcp_write,
      sk_tcp_write_oob,
      sk_tcp_write_eof,
      sk_tcp_flush,
      sk_tcp_set_frozen,
      sk_tcp_socket_error,
      sk_tcp_peer_info,
  };

  DWORD err;
  char *errstr;
  Actual_Socket ret;

  /*
   * Create Socket structure.
   */
  ret = snew(struct Socket_tag);
  ret->fn = &fn_table;
  ret->error = NULL;
  ret->plug = plug;
  bufchain_init(&ret->output_data);
  ret->writable = 1; /* to start with */
  ret->sending_oob = 0;
  ret->outgoingeof = EOF_NO;
  ret->frozen = 1;
  ret->frozen_readable = 0;
  ret->localhost_only = 0; /* unused, but best init anyway */
  ret->pending_error = 0;
  ret->parent = ret->child = NULL;
  ret->addr = NULL;

  ret->s = (SOCKET)ctx.p;

  if (ret->s == INVALID_SOCKET) {
    err = p_WSAGetLastError();
    ret->error = winsock_error_string(err);
    return (Socket)ret;
  }

  ret->oobinline = 0;

  /* Set up a select mechanism. This could be an AsyncSelect on a
   * window, or an EventSelect on an event object. */
  errstr = do_select(ret->s, 1);
  if (errstr) {
    ret->error = errstr;
    return (Socket)ret;
  }

  add234(sktree, ret);

  return (Socket)ret;
}

static DWORD try_connect(Actual_Socket sock)
{
  SOCKET s;
#ifndef NO_IPV6
  SOCKADDR_IN6 a6;
#endif
  SOCKADDR_IN a;
  DWORD err;
  char *errstr;
  short localport;
  int family;

  if (sock->s != INVALID_SOCKET) {
    do_select(sock->s, 0);
    p_closesocket(sock->s);
  }

  {
    struct SockAddr_tag thisaddr = sk_extractaddr_tmp(sock->addr, &sock->step);
    plug_log(sock->plug, 0, &thisaddr, sock->port, NULL, 0);
  }

  /*
   * Open socket.
   */
  family = SOCKADDR_FAMILY(sock->addr, sock->step);

  /*
   * Remove the socket from the tree before we overwrite its
   * internal socket id, because that forms part of the tree's
   * sorting criterion. We'll add it back before exiting this
   * function, whether we changed anything or not.
   */
  del234(sktree, sock);

  s = p_socket(family, SOCK_STREAM, 0);
  sock->s = s;

  if (s == INVALID_SOCKET) {
    err = p_WSAGetLastError();
    sock->error = winsock_error_string(err);
    goto ret;
  }

  SetHandleInformation((HANDLE)s, HANDLE_FLAG_INHERIT, 0);

  if (sock->oobinline) {
    BOOL b = TRUE;
    p_setsockopt(s, SOL_SOCKET, SO_OOBINLINE, (void *)&b, sizeof(b));
  }

  if (sock->nodelay) {
    BOOL b = TRUE;
    p_setsockopt(s, IPPROTO_TCP, TCP_NODELAY, (void *)&b, sizeof(b));
  }

  if (sock->keepalive) {
    BOOL b = TRUE;
    p_setsockopt(s, SOL_SOCKET, SO_KEEPALIVE, (void *)&b, sizeof(b));
  }

  /*
   * Bind to local address.
   */
  if (sock->privport)
    localport = 1023; /* count from 1023 downwards */
  else
    localport = 0; /* just use port 0 (ie winsock picks) */

  /* Loop round trying to bind */
  while (1) {
    int sockcode;

#ifndef NO_IPV6
    if (family == AF_INET6) {
      memset(&a6, 0, sizeof(a6));
      a6.sin6_family = AF_INET6;
      /*a6.sin6_addr = in6addr_any; */ /* == 0 done by memset() */
      a6.sin6_port = p_htons(localport);
    } else
#endif
    {
      a.sin_family = AF_INET;
      a.sin_addr.s_addr = p_htonl(INADDR_ANY);
      a.sin_port = p_htons(localport);
    }
#ifndef NO_IPV6
    sockcode = p_bind(
        s,
        (family == AF_INET6 ? (struct sockaddr *)&a6 : (struct sockaddr *)&a),
        (family == AF_INET6 ? sizeof(a6) : sizeof(a)));
#else
    sockcode = p_bind(s, (struct sockaddr *)&a, sizeof(a));
#endif
    if (sockcode != SOCKET_ERROR) {
      err = 0;
      break; /* done */
    } else {
      err = p_WSAGetLastError();
      if (err != WSAEADDRINUSE) /* failed, for a bad reason */
        break;
    }

    if (localport == 0)
      break; /* we're only looping once */
    localport--;
    if (localport == 0)
      break; /* we might have got to the end */
  }

  if (err) {
    sock->error = winsock_error_string(err);
    goto ret;
  }

  /*
   * Connect to remote address.
   */
#ifndef NO_IPV6
  if (sock->step.ai) {
    if (family == AF_INET6) {
      a6.sin6_family = AF_INET6;
      a6.sin6_port = p_htons((short)sock->port);
      a6.sin6_addr = ((struct sockaddr_in6 *)sock->step.ai->ai_addr)->sin6_addr;
      a6.sin6_flowinfo =
          ((struct sockaddr_in6 *)sock->step.ai->ai_addr)->sin6_flowinfo;
      a6.sin6_scope_id =
          ((struct sockaddr_in6 *)sock->step.ai->ai_addr)->sin6_scope_id;
    } else {
      a.sin_family = AF_INET;
      a.sin_addr = ((struct sockaddr_in *)sock->step.ai->ai_addr)->sin_addr;
      a.sin_port = p_htons((short)sock->port);
    }
  } else
#endif
  {
    assert(sock->addr->addresses &&
           sock->step.curraddr < sock->addr->naddresses);
    a.sin_family = AF_INET;
    a.sin_addr.s_addr = p_htonl(sock->addr->addresses[sock->step.curraddr]);
    a.sin_port = p_htons((short)sock->port);
  }

  /* Set up a select mechanism. This could be an AsyncSelect on a
   * window, or an EventSelect on an event object. */
  errstr = do_select(s, 1);
  if (errstr) {
    sock->error = errstr;
    err = 1;
    goto ret;
  }

  if ((
#ifndef NO_IPV6
          p_connect(s,
                    ((family == AF_INET6) ? (struct sockaddr *)&a6
                                          : (struct sockaddr *)&a),
                    (family == AF_INET6) ? sizeof(a6) : sizeof(a))
#else
          p_connect(s, (struct sockaddr *)&a, sizeof(a))
#endif
              ) == SOCKET_ERROR) {
    err = p_WSAGetLastError();
    /*
     * We expect a potential EWOULDBLOCK here, because the
     * chances are the front end has done a select for
     * FD_CONNECT, so that connect() will complete
     * asynchronously.
     */
    if (err != WSAEWOULDBLOCK) {
      sock->error = winsock_error_string(err);
      goto ret;
    }
  } else {
    /*
     * If we _don't_ get EWOULDBLOCK, the connect has completed
     * and we should set the socket as writable.
     */
    sock->writable = 1;
  }

  err = 0;

ret:

  /*
   * No matter what happened, put the socket back in the tree.
   */
  add234(sktree, sock);

  if (err) {
    struct SockAddr_tag thisaddr = sk_extractaddr_tmp(sock->addr, &sock->step);
    plug_log(sock->plug, 1, &thisaddr, sock->port, sock->error, err);
  }
  return err;
}

Socket sk_new(SockAddr addr,
              int port,
              int privport,
              int oobinline,
              int nodelay,
              int keepalive,
              Plug plug)
{
  static const struct socket_function_table fn_table = {
      sk_tcp_plug,
      sk_tcp_close,
      sk_tcp_write,
      sk_tcp_write_oob,
      sk_tcp_write_eof,
      sk_tcp_flush,
      sk_tcp_set_frozen,
      sk_tcp_socket_error,
      sk_tcp_peer_info,
  };

  Actual_Socket ret;
  DWORD err;

  /*
   * Create Socket structure.
   */
  ret = snew(struct Socket_tag);
  ret->fn = &fn_table;
  ret->error = NULL;
  ret->plug = plug;
  bufchain_init(&ret->output_data);
  ret->connected = 0; /* to start with */
  ret->writable = 0;  /* to start with */
  ret->sending_oob = 0;
  ret->outgoingeof = EOF_NO;
  ret->frozen = 0;
  ret->frozen_readable = 0;
  ret->localhost_only = 0; /* unused, but best init anyway */
  ret->pending_error = 0;
  ret->parent = ret->child = NULL;
  ret->oobinline = oobinline;
  ret->nodelay = nodelay;
  ret->keepalive = keepalive;
  ret->privport = privport;
  ret->port = port;
  ret->addr = addr;
  START_STEP(ret->addr, ret->step);
  ret->s = INVALID_SOCKET;

  err = 0;
  do {
    err = try_connect(ret);
  } while (err && sk_nextaddr(ret->addr, &ret->step));

  return (Socket)ret;
}

Socket sk_newlistener(const char *srcaddr,
                      int port,
                      Plug plug,
                      int local_host_only,
                      int orig_address_family)
{
  static const struct socket_function_table fn_table = {
      sk_tcp_plug,
      sk_tcp_close,
      sk_tcp_write,
      sk_tcp_write_oob,
      sk_tcp_write_eof,
      sk_tcp_flush,
      sk_tcp_set_frozen,
      sk_tcp_socket_error,
      sk_tcp_peer_info,
  };

  SOCKET s;
#ifndef NO_IPV6
  SOCKADDR_IN6 a6;
#endif
  SOCKADDR_IN a;

  DWORD err;
  char *errstr;
  Actual_Socket ret;
  int retcode;
  int on = 1;

  int address_family;

  /*
   * Create Socket structure.
   */
  ret = snew(struct Socket_tag);
  ret->fn = &fn_table;
  ret->error = NULL;
  ret->plug = plug;
  bufchain_init(&ret->output_data);
  ret->writable = 0; /* to start with */
  ret->sending_oob = 0;
  ret->outgoingeof = EOF_NO;
  ret->frozen = 0;
  ret->frozen_readable = 0;
  ret->localhost_only = local_host_only;
  ret->pending_error = 0;
  ret->parent = ret->child = NULL;
  ret->addr = NULL;

  /*
   * Translate address_family from platform-independent constants
   * into local reality.
   */
  address_family = (orig_address_family == ADDRTYPE_IPV4
                        ? AF_INET
                        :
#ifndef NO_IPV6
                        orig_address_family == ADDRTYPE_IPV6 ? AF_INET6 :
#endif
                                                             AF_UNSPEC);

  /*
   * Our default, if passed the `don't care' value
   * ADDRTYPE_UNSPEC, is to listen on IPv4. If IPv6 is supported,
   * we will also set up a second socket listening on IPv6, but
   * the v4 one is primary since that ought to work even on
   * non-v6-supporting systems.
   */
  if (address_family == AF_UNSPEC)
    address_family = AF_INET;

  /*
   * Open socket.
   */
  s = p_socket(address_family, SOCK_STREAM, 0);
  ret->s = s;

  if (s == INVALID_SOCKET) {
    err = p_WSAGetLastError();
    ret->error = winsock_error_string(err);
    return (Socket)ret;
  }

  SetHandleInformation((HANDLE)s, HANDLE_FLAG_INHERIT, 0);

  ret->oobinline = 0;

  p_setsockopt(s, SOL_SOCKET, SO_REUSEADDR, (const char *)&on, sizeof(on));

#ifndef NO_IPV6
  if (address_family == AF_INET6) {
    memset(&a6, 0, sizeof(a6));
    a6.sin6_family = AF_INET6;
    if (local_host_only)
      a6.sin6_addr = in6addr_loopback;
    else
      a6.sin6_addr = in6addr_any;
    if (srcaddr != NULL && p_getaddrinfo) {
      struct addrinfo hints;
      struct addrinfo *ai;
      int err;

      memset(&hints, 0, sizeof(hints));
      hints.ai_family = AF_INET6;
      hints.ai_flags = 0;
      {
        /* strip [] on IPv6 address literals */
        char *trimmed_addr = host_strduptrim(srcaddr);
        err = p_getaddrinfo(trimmed_addr, NULL, &hints, &ai);
        sfree(trimmed_addr);
      }
      if (err == 0 && ai->ai_family == AF_INET6) {
        a6.sin6_addr = ((struct sockaddr_in6 *)ai->ai_addr)->sin6_addr;
      }
    }
    a6.sin6_port = p_htons(port);
  } else
#endif
  {
    int got_addr = 0;
    a.sin_family = AF_INET;

    /*
     * Bind to source address. First try an explicitly
     * specified one...
     */
    if (srcaddr) {
      a.sin_addr.s_addr = p_inet_addr(srcaddr);
      if (a.sin_addr.s_addr != INADDR_NONE) {
        /* Override localhost_only with specified listen addr. */
        ret->localhost_only = ipv4_is_loopback(a.sin_addr);
        got_addr = 1;
      }
    }

    /*
     * ... and failing that, go with one of the standard ones.
     */
    if (!got_addr) {
      if (local_host_only)
        a.sin_addr.s_addr = p_htonl(INADDR_LOOPBACK);
      else
        a.sin_addr.s_addr = p_htonl(INADDR_ANY);
    }

    a.sin_port = p_htons((short)port);
  }
#ifndef NO_IPV6
  retcode = p_bind(s,
                   (address_family == AF_INET6 ? (struct sockaddr *)&a6
                                               : (struct sockaddr *)&a),
                   (address_family == AF_INET6 ? sizeof(a6) : sizeof(a)));
#else
  retcode = p_bind(s, (struct sockaddr *)&a, sizeof(a));
#endif
  if (retcode != SOCKET_ERROR) {
    err = 0;
  } else {
    err = p_WSAGetLastError();
  }

  if (err) {
    p_closesocket(s);
    ret->error = winsock_error_string(err);
    return (Socket)ret;
  }

  if (p_listen(s, SOMAXCONN) == SOCKET_ERROR) {
    p_closesocket(s);
    ret->error = winsock_error_string(p_WSAGetLastError());
    return (Socket)ret;
  }

  /* Set up a select mechanism. This could be an AsyncSelect on a
   * window, or an EventSelect on an event object. */
  errstr = do_select(s, 1);
  if (errstr) {
    p_closesocket(s);
    ret->error = errstr;
    return (Socket)ret;
  }

  add234(sktree, ret);

#ifndef NO_IPV6
  /*
   * If we were given ADDRTYPE_UNSPEC, we must also create an
   * IPv6 listening socket and link it to this one.
   */
  if (address_family == AF_INET && orig_address_family == ADDRTYPE_UNSPEC) {
    Actual_Socket other;

    other = (Actual_Socket)sk_newlistener(
        srcaddr, port, plug, local_host_only, ADDRTYPE_IPV6);

    if (other) {
      if (!other->error) {
        other->parent = ret;
        ret->child = other;
      } else {
        sfree(other);
      }
    }
  }
#endif

  return (Socket)ret;
}

static void sk_tcp_close(Socket sock)
{
  extern char *do_select(SOCKET skt, int startup);
  Actual_Socket s = (Actual_Socket)sock;

  if (s->child)
    sk_tcp_close((Socket)s->child);

  del234(sktree, s);
  do_select(s->s, 0);
  p_closesocket(s->s);
  if (s->addr)
    sk_addr_free(s->addr);
  sfree(s);
}

/*
 * Deal with socket errors detected in try_send().
 */
static void socket_error_callback(void *vs)
{
  Actual_Socket s = (Actual_Socket)vs;

  /*
   * Just in case other socket work has caused this socket to vanish
   * or become somehow non-erroneous before this callback arrived...
   */
  if (!find234(sktree, s, NULL) || !s->pending_error)
    return;

  /*
   * An error has occurred on this socket. Pass it to the plug.
   */
  plug_closing(
      s->plug, winsock_error_string(s->pending_error), s->pending_error, 0);
}

/*
 * The function which tries to send on a socket once it's deemed
 * writable.
 */
void try_send(Actual_Socket s)
{
  while (s->sending_oob || bufchain_size(&s->output_data) > 0) {
    int nsent;
    DWORD err;
    void *data;
    int len, urgentflag;

    if (s->sending_oob) {
      urgentflag = MSG_OOB;
      len = s->sending_oob;
      data = &s->oobdata;
    } else {
      urgentflag = 0;
      bufchain_prefix(&s->output_data, &data, &len);
    }
    nsent = p_send(s->s, data, len, urgentflag);
    noise_ultralight(nsent);
    if (nsent <= 0) {
      err = (nsent < 0 ? p_WSAGetLastError() : 0);
      if ((err < WSABASEERR && nsent < 0) || err == WSAEWOULDBLOCK) {
        /*
         * Perfectly normal: we've sent all we can for the moment.
         *
         * (Some WinSock send() implementations can return
         * <0 but leave no sensible error indication -
         * WSAGetLastError() is called but returns zero or
         * a small number - so we check that case and treat
         * it just like WSAEWOULDBLOCK.)
         */
        s->writable = FALSE;
        return;
      } else if (nsent == 0 || err == WSAECONNABORTED || err == WSAECONNRESET) {
        /*
         * If send() returns CONNABORTED or CONNRESET, we
         * unfortunately can't just call plug_closing(),
         * because it's quite likely that we're currently
         * _in_ a call from the code we'd be calling back
         * to, so we'd have to make half the SSH code
         * reentrant. Instead we flag a pending error on
         * the socket, to be dealt with (by calling
         * plug_closing()) at some suitable future moment.
         */
        s->pending_error = err;
        queue_toplevel_callback(socket_error_callback, s);
        return;
      } else {
        /* We're inside the Windows frontend here, so we know
         * that the frontend handle is unnecessary. */
        logevent(NULL, winsock_error_string(err));
        fatalbox("%s", winsock_error_string(err));
      }
    } else {
      if (s->sending_oob) {
        if (nsent < len) {
          memmove(s->oobdata, s->oobdata + nsent, len - nsent);
          s->sending_oob = len - nsent;
        } else {
          s->sending_oob = 0;
        }
      } else {
        bufchain_consume(&s->output_data, nsent);
      }
    }
  }

  /*
   * If we reach here, we've finished sending everything we might
   * have needed to send. Send EOF, if we need to.
   */
  if (s->outgoingeof == EOF_PENDING) {
    p_shutdown(s->s, SD_SEND);
    s->outgoingeof = EOF_SENT;
  }
}

static int sk_tcp_write(Socket sock, const char *buf, int len)
{
  Actual_Socket s = (Actual_Socket)sock;

  assert(s->outgoingeof == EOF_NO);

  /*
   * Add the data to the buffer list on the socket.
   */
  bufchain_add(&s->output_data, buf, len);

  /*
   * Now try sending from the start of the buffer list.
   */
  if (s->writable)
    try_send(s);

  return bufchain_size(&s->output_data);
}

static int sk_tcp_write_oob(Socket sock, const char *buf, int len)
{
  Actual_Socket s = (Actual_Socket)sock;

  assert(s->outgoingeof == EOF_NO);

  /*
   * Replace the buffer list on the socket with the data.
   */
  bufchain_clear(&s->output_data);
  assert(len <= sizeof(s->oobdata));
  memcpy(s->oobdata, buf, len);
  s->sending_oob = len;

  /*
   * Now try sending from the start of the buffer list.
   */
  if (s->writable)
    try_send(s);

  return s->sending_oob;
}

static void sk_tcp_write_eof(Socket sock)
{
  Actual_Socket s = (Actual_Socket)sock;

  assert(s->outgoingeof == EOF_NO);

  /*
   * Mark the socket as pending outgoing EOF.
   */
  s->outgoingeof = EOF_PENDING;

  /*
   * Now try sending from the start of the buffer list.
   */
  if (s->writable)
    try_send(s);
}

void select_result(WPARAM wParam, LPARAM lParam)
{
  int ret;
  DWORD err;
  char buf[20480]; /* nice big buffer for plenty of speed */
  Actual_Socket s;
  u_long atmark;

  /* wParam is the socket itself */

  if (wParam == 0)
    return; /* boggle */

  s = find234(sktree, (void *)wParam, cmpforsearch);
  if (!s)
    return; /* boggle */

  if ((err = WSAGETSELECTERROR(lParam)) != 0) {
    /*
     * An error has occurred on this socket. Pass it to the
     * plug.
     */
    if (s->addr) {
      struct SockAddr_tag thisaddr = sk_extractaddr_tmp(s->addr, &s->step);
      plug_log(s->plug, 1, &thisaddr, s->port, winsock_error_string(err), err);
      while (err && s->addr && sk_nextaddr(s->addr, &s->step)) {
        err = try_connect(s);
      }
    }
    if (err != 0)
      plug_closing(s->plug, winsock_error_string(err), err, 0);
    return;
  }

  noise_ultralight(lParam);

  switch (WSAGETSELECTEVENT(lParam)) {
  case FD_CONNECT:
    s->connected = s->writable = 1;
    /*
     * Once a socket is connected, we can stop falling
     * back through the candidate addresses to connect
     * to.
     */
    if (s->addr) {
      sk_addr_free(s->addr);
      s->addr = NULL;
    }
    break;
  case FD_READ:
    /* In the case the socket is still frozen, we don't even bother */
    if (s->frozen) {
      s->frozen_readable = 1;
      break;
    }

    /*
     * We have received data on the socket. For an oobinline
     * socket, this might be data _before_ an urgent pointer,
     * in which case we send it to the back end with type==1
     * (data prior to urgent).
     */
    if (s->oobinline) {
      atmark = 1;
      p_ioctlsocket(s->s, SIOCATMARK, &atmark);
      /*
       * Avoid checking the return value from ioctlsocket(),
       * on the grounds that some WinSock wrappers don't
       * support it. If it does nothing, we get atmark==1,
       * which is equivalent to `no OOB pending', so the
       * effect will be to non-OOB-ify any OOB data.
       */
    } else
      atmark = 1;

    ret = p_recv(s->s, buf, sizeof(buf), 0);
    noise_ultralight(ret);
    if (ret < 0) {
      err = p_WSAGetLastError();
      if (err == WSAEWOULDBLOCK) {
        break;
      }
    }
    if (ret < 0) {
      plug_closing(s->plug, winsock_error_string(err), err, 0);
    } else if (0 == ret) {
      plug_closing(s->plug, NULL, 0, 0);
    } else {
      plug_receive(s->plug, atmark ? 0 : 1, buf, ret);
    }
    break;
  case FD_OOB:
    /*
     * This will only happen on a non-oobinline socket. It
     * indicates that we can immediately perform an OOB read
     * and get back OOB data, which we will send to the back
     * end with type==2 (urgent data).
     */
    ret = p_recv(s->s, buf, sizeof(buf), MSG_OOB);
    noise_ultralight(ret);
    if (ret <= 0) {
      const char *str = (ret == 0 ? "Internal networking trouble"
                                  : winsock_error_string(p_WSAGetLastError()));
      /* We're inside the Windows frontend here, so we know
       * that the frontend handle is unnecessary. */
      logevent(NULL, str);
      fatalbox("%s", str);
    } else {
      plug_receive(s->plug, 2, buf, ret);
    }
    break;
  case FD_WRITE: {
    int bufsize_before, bufsize_after;
    s->writable = 1;
    bufsize_before = s->sending_oob + bufchain_size(&s->output_data);
    try_send(s);
    bufsize_after = s->sending_oob + bufchain_size(&s->output_data);
    if (bufsize_after < bufsize_before)
      plug_sent(s->plug, bufsize_after);
  } break;
  case FD_CLOSE:
    /* Signal a close on the socket. First read any outstanding data. */
    do {
      ret = p_recv(s->s, buf, sizeof(buf), 0);
      if (ret < 0) {
        err = p_WSAGetLastError();
        if (err == WSAEWOULDBLOCK)
          break;
        plug_closing(s->plug, winsock_error_string(err), err, 0);
      } else {
        if (ret)
          plug_receive(s->plug, 0, buf, ret);
        else
          plug_closing(s->plug, NULL, 0, 0);
      }
    } while (ret > 0);
    return;
  case FD_ACCEPT: {
#ifdef NO_IPV6
    struct sockaddr_in isa;
#else
    struct sockaddr_storage isa;
#endif
    int addrlen = sizeof(isa);
    SOCKET t; /* socket of connection */
    accept_ctx_t actx;

    memset(&isa, 0, sizeof(isa));
    err = 0;
    t = p_accept(s->s, (struct sockaddr *)&isa, &addrlen);
    if (t == INVALID_SOCKET) {
      err = p_WSAGetLastError();
      if (err == WSATRY_AGAIN)
        break;
    }

    actx.p = (void *)t;

#ifndef NO_IPV6
    if (isa.ss_family == AF_INET && s->localhost_only &&
        !ipv4_is_local_addr(((struct sockaddr_in *)&isa)->sin_addr))
#else
    if (s->localhost_only && !ipv4_is_local_addr(isa.sin_addr))
#endif
    {
      p_closesocket(t); /* dodgy WinSock let nonlocal through */
    } else if (plug_accepting(s->plug, sk_tcp_accept, actx)) {
      p_closesocket(t); /* denied or error */
    }
  }
  }
}

/*
 * Special error values are returned from sk_namelookup and sk_new
 * if there's a problem. These functions extract an error message,
 * or return NULL if there's no problem.
 */
const char *sk_addr_error(SockAddr addr)
{
  return addr->error;
}
static const char *sk_tcp_socket_error(Socket sock)
{
  Actual_Socket s = (Actual_Socket)sock;
  return s->error;
}

static char *sk_tcp_peer_info(Socket sock)
{
  Actual_Socket s = (Actual_Socket)sock;
#ifdef NO_IPV6
  struct sockaddr_in addr;
#else
  struct sockaddr_storage addr;
  char buf[INET6_ADDRSTRLEN];
#endif
  int addrlen = sizeof(addr);

  if (p_getpeername(s->s, (struct sockaddr *)&addr, &addrlen) < 0)
    return NULL;

  if (((struct sockaddr *)&addr)->sa_family == AF_INET) {
    return dupprintf("%s:%d",
                     p_inet_ntoa(((struct sockaddr_in *)&addr)->sin_addr),
                     (int)p_ntohs(((struct sockaddr_in *)&addr)->sin_port));
#ifndef NO_IPV6
  } else if (((struct sockaddr *)&addr)->sa_family == AF_INET6) {
    return dupprintf("[%s]:%d",
                     p_inet_ntop(AF_INET6,
                                 &((struct sockaddr_in6 *)&addr)->sin6_addr,
                                 buf,
                                 sizeof(buf)),
                     (int)p_ntohs(((struct sockaddr_in6 *)&addr)->sin6_port));
#endif
  } else {
    return NULL;
  }
}

static void sk_tcp_set_frozen(Socket sock, int is_frozen)
{
  Actual_Socket s = (Actual_Socket)sock;
  if (s->frozen == is_frozen)
    return;
  s->frozen = is_frozen;
  if (!is_frozen) {
    do_select(s->s, 1);
    if (s->frozen_readable) {
      char c;
      p_recv(s->s, &c, 1, MSG_PEEK);
    }
  }
  s->frozen_readable = 0;
}

void socket_reselect_all(void)
{
  Actual_Socket s;
  int i;

  for (i = 0; (s = index234(sktree, i)) != NULL; i++) {
    if (!s->frozen)
      do_select(s->s, 1);
  }
}

/*
 * For Plink: enumerate all sockets currently active.
 */
SOCKET first_socket(int *state)
{
  Actual_Socket s;
  *state = 0;
  s = index234(sktree, (*state)++);
  return s ? s->s : INVALID_SOCKET;
}

SOCKET next_socket(int *state)
{
  Actual_Socket s = index234(sktree, (*state)++);
  return s ? s->s : INVALID_SOCKET;
}

extern int socket_writable(SOCKET skt)
{
  Actual_Socket s = find234(sktree, (void *)skt, cmpforsearch);

  if (s)
    return bufchain_size(&s->output_data) > 0;
  else
    return 0;
}

int net_service_lookup(char *service)
{
  struct servent *se;
  se = p_getservbyname(service, NULL);
  if (se != NULL)
    return p_ntohs(se->s_port);
  else
    return 0;
}

char *get_hostname(void)
{
  int len = 128;
  char *hostname = NULL;
  do {
    len *= 2;
    hostname = sresize(hostname, len, char);
    if (p_gethostname(hostname, len) < 0) {
      sfree(hostname);
      hostname = NULL;
      break;
    }
  } while (strlen(hostname) >= (size_t)(len - 1));
  return hostname;
}

SockAddr platform_get_x11_unix_address(const char *display,
                                       int displaynum,
                                       char **canonicalname)
{
  SockAddr ret = snew(struct SockAddr_tag);
  memset(ret, 0, sizeof(struct SockAddr_tag));
  ret->error = "unix sockets not supported on this platform";
  ret->refcount = 1;
  return ret;
}
